1. Field of the Invention
This invention relates to gas atomizing nozzles, and a method and apparatus for varying and controlling the degree of atomization, the nozzle capacity and the spray dilution, over wide ranges.
2. Description of the Prior Art
Atomization is considered to be the process of breaking up a liquid and dispersing it into a surrounding atmosphere in the form of fog, mist, fine spray or coarse drops. Gas atomization involves the break-up of a liquid stream by contact with a high velocity gas stream, typically compressed air or steam. Industrially, gas atomizing nozzles are generally employed where relatively fine sprays are required. Typically, the degree of atomization, with gas atomizing nozzles, is such that the characteristic droplet size of the resulting spray (frequently expressed in terms of the mass median diameter, or MMD) is in the range of 10 to 100 microns, and the individual nozzle capacities are usually below 1 gpm. (4 lit./min.).
Many techniques have been devised in an effort to significantly increase the liquid capacity, and to economically apply gas atomizing nozzles to processes in which MMD's greater than 100 microns are permissible. A brief outline of some of these follows: Multiple gas jets, set at an angle to the liquid jet, have been used to produce jet impact. A spiral insert, or tangential liquid entry, placed upstream of a liquid orifice, has been used to produce a diverging liquid sheet. Opposing, tangential velocity components have been added to the gas stream. The liquid has been fed through a converging annular nozzle so that it flows with a radially inward component, as a sheet, into a centrally located gas nozzle. Mixing chambers, usually terminating in a nozzle, have been added downstream of the liquid and primary gas contact zone. Convergent-divergent gas nozzles have been used in an effort to aid atomization by supersonic flow or shock wave effects. The general problem with prior efforts to increase gas atomizer capacity is that the spray droplet size increases as the flow rate and nozzle size are increased, and the gas consumption becomes excessive. Because of the difficulties encountered in scaling up gas atomizers, pressure nozzles, spining disk atomizers, or a multiplicity of gas atomizers, are generally employed where high flow rates are required.
One exception has been the field of snowmaking on ski slopes where relatively large compressed air nozzles are employed to atomize water. In this application, the compressed air serves the additional purpose of diluting the spray plume by atmospheric entrainment with the large volumes of cold ambient air required to freeze the droplets. At low temperatures, relatively large droplets and relatively small volumes of compressed air may be employed, with the result that nozzle capacities in excess of 100 gpm. (400 lit./min.) have been attained. As the ambient wet bulb temperature increases, the droplet size requirements and liquid capacities rapidly decrease, and the air requirements increase, so that snowmaking operation becomes uneconomical much above 20 degrees F. (-7 C.).
Many atomization applications require a thorough and rapid intermixing with a large volume of secondary, or ambient gas. These include spray cooling of water, spray drying, combustion and spray washing. Forced draft blowers are often used for intermixing the spray and atmosphere. Because of the atmospheric entrainment produced by gas atomization, the prospect of its application becomes attractive if large liquid flow rates can be attained with control of the degree of atomization over a wide range of droplet sizes, with adequate spray dillution, and with economical power consumption.